1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic recording medium, which includes, in particular, a featured step of forming a lubricant layer.
2. Description of the Related Art
Fixed magnetic recording media are used for data recording in computers. A fixed magnetic recording medium apparatus has a mechanism for driving a magnetic recording medium, a mechanism for driving a magnetic head, a mechanism for stopping the magnetic head, and a mechanism for controlling data transmission. The fixed magnetic recording medium apparatus also has one or more magnetic recording media installed in the apparatus. Fixed magnetic recording medium apparatuses in recent years have progressed remarkably towards higher recording density, larger capacity, and faster data transmission rate.
In a conventional fixed magnetic recording medium apparatus, a CSS system (contact start and stop system) is employed, in which the magnetic head flies when the magnetic recording medium rotates, and comes in contact with the surface of the magnetic recording medium when the rotation driving motor for rotating the magnetic recording medium stops. In this system, the magnetic head slides on the surface of the magnetic recording medium in the processes of starting and stopping the rotation of the magnetic recording medium, and generates friction between the head and the medium, thereby causing damage in the magnetic layer of the magnetic recording medium. In order to protect the magnetic layer against the damage due to friction, a protective layer is formed on the surface of the magnetic layer, and further, a lubricant layer is formed on the protective layer for improving surface lubricating performance.
The lubricant layer for improving surface lubricating performance of a magnetic recording medium must be formed stably on the surface of the protective layer with a uniform thickness. In addition, it is important that the lubricant layer adheres firmly and bonds strongly to the protective layer. A type of lubricant used for obtaining the firm adhesiveness is a perfluoropolyether lubricant having an end group or groups of a hydroxyl group or a piperonyl group. Specific examples of the perfluoropolyether lubricant include “Fomblin Z DOL” and “AM2001”, both being products of Solvey Solexis Company. For many of the perfluoropolyether lubricants used for conventional magnetic recording media, the lubricating performance and heat-resisting property deteriorate if the molecular weight of the perfluoropolyether lubricant is too small, or, if the molecular weight is too large, the magnetic recording media are liable to adsorb. Accordingly, a number-averaged molecular weight Mn in the range of 1,000 to 10,000 preferably is used. It is yet possible to reduce ill effects of adsorption and poor heat resistance by narrowing the molecular weight distribution range.
The dip-coating method, which is one of the known methods for forming a lubricant layer, is widely used. In the process of coating with a perfluoropolyether lubricant, a lubricant solution is generally used with a fluorine-containing solvent. The selection of the solvent is a crucial issue in this process, in view of the effects of the solvent on the environment and the cost. The boiling point of the solvent is one of the restricting conditions in the selection of the solvent. A solvent having a boiling point of about 60° C. is generally used, and a lubricant concentration is several hundred ppm in weight in most cases.
In a magnetic recording medium after applying the perfluoropolyether lubricant, a thick area (of lubricant accumulation) may be formed at a spot of contact with the hanger for supporting the magnetic recording medium in the application process. The thick area degrades the uniformity of the thickness of the medium.
The spin-coating method, another known technique uncommon today, can also be used for coating a magnetic recording medium with the lubricant. The technique uses a solvent with a high boiling point (over 80° C.) and forms a lubricant layer by making droplets of coating liquid falling through a nozzle onto the rotating magnetic recording medium.
Spraying is also known as a coating technique in other technical fields. Japanese Unexamined Patent Application Publication No. H05-189761 discloses a method of spraying a mist of lubricant without using a fluorine-containing gas. Japanese Unexamined Patent Application Publication No. H08-161739 discloses an apparatus for forming a lubricant layer by spraying droplets of lubricant of a uniform size using ultrasonic vibration.
Japanese Unexamined Patent Application Publication No. 2004-246985 discloses an ink jet method for coating with lubricant. This method uses a low viscosity lubricant, and a lubricant layer is formed by self leveling at an ambient temperature.
The technique for forming a lubricant layer by means of a dip-coating method uses a solvent with a low boiling point of about 60° C. for preparing the lubricant solution. As a result, there is a risk of large load on the environment caused by the evaporation of the solvent during the storage and application process of the lubricant solution.
Moreover, it is also feared, as described earlier, that accumulation of lubricant in the coating process hinders the effort of reducing the flying height of the magnetic head.
Since the technique for forming a lubricant layer by means of a spin coating method utilizes diffusion of the solution by a centrifugal force, it is difficult to control the process conditions to accomplish a uniform film thickness, with a difference of several Angstroms between the inner and outer circumferences. Further, since the selection of the solvent is very limited, it is difficult to apply a lubricant having a low solubility like “Fomblin Z Tetraol” (a product of Solvey Solexis Company). There is an additional problem that the heat of vaporization is necessary for vaporizing the solvent, which cools the coated magnetic recording medium, causing condensation of moisture in the air.
In the technique for forming a lubricant layer by means of the spray method as disclosed in Japanese Unexamined Patent Application Publications No. H05-189761 and No. H08-161739, the concentration of the lubricant solution is about 0.05 wt %, which is almost the same value as the one in the dip-coating method employed in magnetic recording media. Consequently, it is hard to reduce the amount of solvent consumption. It is yet difficult in these spray methods, in which the solution is once transformed into mist, to form a lubricant layer having a desired thickness in a desired narrow region, or to form a lubricant layer having a selected region composed of a different type of lubricant agent from the lubricant in the other regions.
Japanese Unexamined Patent Application Publication No. 2004-246985 presumes the use of low viscosity lubricant, it is thus impossible to use a lubricant with a high viscosity (higher than 1,000 mPa-s at 20° C.) like “Fomblin Z Tetraol” (a product of Solvey Solexis Company). This method forms a lubricant layer with self-leveling at an ambient temperature, and there is no means for improving adhesiveness of the lubricant layer.
In a conventional process of forming a lubricant layer, a magnetic recording medium coated with a lubricant layer is then subjected to a step of heating or UV irradiation in order to improve the adhesiveness between the lubricant layer and the surface of the magnetic recording medium. A heating step in particular, needs to be carried out at a temperature in a range of 70° C. to 150° C. for a time duration of from 15 minutes to 2 hours. Thus, a conventional process of forming a lubricant layer is a time-consuming step in the process of manufacturing a magnetic recording medium.